The present invention relates to a document handler for a copier and more particularly to an improved document handler utilizing a plurality of low friction unapertured belts in a vacuum transport system, where the improvement relates to the optimization of the optical properties of the document handler.
A particular problem with modern copiers with short conjugate optics is maintaining the document being copied sufficiently flat on the imaging platen to avoid optical distortion and to maintain all of the document in focus. This is particularly difficult to achieve while maintaining positional control over the document, especially when rapidly replacing the document with another document to be copied. An efficient means for copying documents in this type of system is a multi-belt vacuum document transport system. An illustrative example of such a system is disclosed in U.S. Pat. No. 4,294,540 issued on Oct. 13, 1981 and assigned to Xerox Corporation. This patent discloses a vacuum belt platen transport system which comprises plural belts moving under a white vacuum plenum backing surface overlying the platen and includes a vacuum source for applying a partial vacuum to a document sheet being transported by the moving belts. Other patents disclosing plural belts in a vacuum transport system are U.S. Pat. No. 4,412,738 issued on Nov. 1, 1983 to Ahern et al. and U.S. Pat. No. 4,589,652 issued on May 20, 1986 to Silverberg.
These multiple belt transports are subject to copy defect problems which can be described as "show-around" and "show-through". "Show-through" is the printing out of dark areas on the copy sheet because the copier optics "sees" dark areas on the document transport through the document, particularly through a transparent or very thin or otherwise translucent document. "Show-around" can occur when the document is mis-registered, or a reduction copy is being made, which results in areas of the platen transport beyond (outside of) one or more edges of the original being directly imaged onto the copy. Both of these types of copy defects are more prevalent with vacuum platen transports and plural belt platen transports, because the belt edges and vacuum apertures tend to have both printable edge shadows and dirt contamination (visible darkening), particularly if such belts are of a relatively thick or high friction elastometric material. This can produce very undesirable dark areas on the copy sheets.
The present invention is directed towards a plural belt document feeder whose various components have been optimized to reduce or eliminate "show-through" and "show-around" problems. The reflectance and optical characteristics of the belt were analyzed and balanced against the reflectivity and surface texture of the plenum to arrive at an optimum balance of all these factors for a wide range of papers. More particularly the invention is directed towards the optimum optical properties of the document feeder which conveys document sheets into and away from an imaging position on a platen of a copier, including a vacuum belt transport system which comprises:
a plurality of spaced-apart belts moving between a plenum having a white backing surface and the platen, and PA1 a vacuum source for applying a partial vacuum to the inter-belt gaps to hold a document sheet against said belts so that the document is conveyed with the movement of the belts, characterized in that the plenum backing surface has a reflectance of approximately 92%, an average surface roughness of approximately 3.6 microns and said belts when backed by the plenum, have a reflectance of approximately 91%.